The present invention relates to the production of a network of fractures in a deep underground segment of rock by means of explosives, e.g., to prepare deep ore bodies for in situ leaching.
Processes for fracturing deep rock are becoming increasingly important as it becomes necessary to tap deep mineralized rock masses, e.g., ore bodies or oil or gas reservoirs located from about 100 feet to about a few thousand feet beneath the earth's surface, in order to supplement or replace dwindling energy sources and minerals supplies. Numerous deposits of ore, for example ore containing copper, nickel, or silver, lie too deep to mine by open-pit methods or are too low in grade to mine by underground methods. Open-pit methods incur both the costs and the environmental impact associated with moving large quantities of earth and rock. Underground methods incur unusually high costs per unit volume of ore mined, as well as difficult safety problems. In contrast, the leaching of ore in place circumvents these difficulties and therefore can be a preferred technique for winning values from some ores that are unsuitable, or marginally suitable, for working by traditional mining methods.
Usually however, ore that is favorably situated for leaching in place has such a large fragment size and such low permeability to leaching solutions that the leaching rate would be too low to support a commercial leaching operation. In such cases, it becomes necessary to prepare the ore for leaching, by fragmenting it in a manner such as to provide the necessary permeability and leachability. The use of explosives to fracture underground segments of mineralized rock to create areas of high permeability has often been suggested. In an oil- or gas-bearing formation the fracturing is required to increase the overall drainage area exposed to the bore of a well penetrating the formation, and thus increase the rate at which hydrocarbon fluids drain toward the well. In an ore body the fracturing is required to increase the surface area of ore accessible to an injected lixiviant, and thus increase the leachability.
The use of nuclear explosives has been proposed for fracturing large-volume, deep ore bodies for subsequent in situ leaching. Also, the use of multiple chemical explosive charges in deep reservoir rock has been described in a method for stimulating hydrocarbon-bearing rock, e.g., in U.S. Pat. No. 3,674,089. However, if a deep ore body, i.e., one lying at depths of about from 100 to 3000 feet from the surface, is to be effectively leached in place, and the ore prepared for leaching by blasting, i.e., blasting in the absence of a free face for the ore to swell toward, it becomes necessary to employ special blasting and associated techniques which will provide and maintain the type of fracture network required for efficient leaching.
The leachability of a fractured ore body depends on the size of the ore fragments, and on the permeability of the intact ore as well as of the fracture system separating the fragments. The permeability of the fracture system separating the fragments, which is variable and generally much higher than the permeability of a single fragment, is determined by a network of wider, open fractures (determining the permeability of the ore body as a whole), and a network of narrower, open fractures (determining the irrigability of individual particles to be leached). Therefore, in explosively fracturing a segment of an ore body to prepare it properly for in situ leaching, the objective is not simply an indiscriminate reduction in the fragment size of the ore body. Smaller-size, well-irrigated fragments have a higher leaching rate than larger-size fragments, but fragment-size reduction by means of blasting processes heretofore known to the art, when applied to deep ore, tends to leave large unbroken fragments of rock, or to create a network of fractures that are largely closed or plugged with fines. An explosive fracturing process is needed which reduces the larger fragments to a size that will leach at an economically acceptable rate, and that will result in a network of open fractures throughout the blasted ore that will permit it to be well-irrigated with leach liquid.